Most electronic equipment having substantial power requirements must draw power from an AC line source. An AC voltage drawn therefrom, however, is generally not useable by the electronic equipment. The AC voltage is therefore converted to a DC voltage by an AC-to-DC converter. Furthermore, most electronic equipment requires either a different DC voltage magnitude other than an associated converted DC voltage magnitude and/or several different DC voltage magnitudes. The different DC voltage magnitudes are subsequently provided by a DC-to-DC converter coupled to an output of the AC-to-DC converter.
Conventional switch-mode power supplies use a diode bridge as a front-end AC-to-DC converter for rectifying the AC voltage. The rectified AC voltage typically contains an unacceptable amount of ripple. This ripple voltage is substantially removed by a large filter capacitor following the diode bridge to provide a DC voltage known as the DC bulk voltage. FIG. 1a depicts a conventional switch-mode power supply having a diode bridge 2 and a filter capacitor 3 for providing the DC bulk voltage. The DC bulk voltage is then provided to a DC-to-DC converter 4 which produces an output voltage at the desired level. Typical DC-to-DC converters rely upon the storage characteristics of capacitors and inductors while alternately switching the applied DC voltage. The result is a squarewave waveform that is rectified by power Schottky diodes and filtered by a capacitor. An improved DC-to-DC converter is described by Andresen in a commonly assigned patent application Ser. No. 07/863,620, filed on an even date herewith.
Several problems exist in conventional switch-mode power supplies. Bridge diodes due to inherent forward voltages cause unwanted power dissipation thereby adversely affecting a conversion efficiency of the power supplies. Furthermore since an input current only conducts during a portion of any cycle, undesirable harmonic distortion occurs (Fourier harmonics). A typical input current waveform 6 is shown in FIG. 1b. As a result of a non-sinusoidal current which has to be provided by a utility source, a power factor is reduced (the ratio of the average power to the magnitude of the complex power), and still more harmonics currents are generated. The result is inefficient use of the input power, transformer and circuit breaker stress and excessive neutral conductor currents in wye-delta four-conductor three-phase circuits.
These problems can be better understood by analyzing a building's electrical distribution system. The power distribution system for a building generally includes a three-phase power input from high voltage lines to a step-down transformer. The step-down transformer's primary is usually connected in a delta configuration which eliminates the need for a fourth neutral conductor and the secondary, connected in the wye configuration (having a common neutral conductor), usually provides 120 volts RMS phase to-neutral (U.S.). Not uncommonly, the loads connected to the three phases of the secondary include appliances, computers, lighting air-conditioning, etc. Equipment using typical switch-mode power supplies present nonlinear loads to the secondary which results in power losses, noise and harmonics, and overstressing.
The diode bridge 2 and filter capacitor 3, as depicted in FIG. 1a present a nonlinear load. The non-sinusoidal current 6, as shown in FIG. lb, creates still further problems in three phase power systems. Ideally the neutral conductor should carry zero current, hence neutral conductors are generally much smaller in diameter than phase conductors. The third harmonics of the non-sinusoidal currents, which usually dominate, are additive. Therefore, a building wired for mainly linear loads, but housing many nonlinear loads, is probably overloading the neutral conductor. The result is a fire hazard.
The input current and power factor of a power supply can be corrected, to some degree, by passive filters. Unfortunately, passive components making up the passive filters produce power dissipation and electromagnetic interface (EMI). The passive components are generally large and expensive, thereby limiting passive power factor correction to utilities or large users and suppliers of electric power. Additionally the drawbacks of passive power factor correction make this solution unsuitable for most electronic equipment.
A more suitable solution for correcting the power factor in electronic equipment involves active circuits and converters. Flyback and boost converter control circuits, for example, are available in integrated circuit form and are capable of providing performance above 0.90 power factor while reducing total harmonic distortion (THD) to below about 5 percent. Boost converters, currently the most popular technique, produce output voltages having greater magnitudes than the input voltages (for example, 120 vac input and 380 vdc output). Boost converters can currently operate effectively up to about 1000 watts. Flyback converters are typically limited to power requirements below about 100 watts. Alternatively, a bulkless converter is able to provide performance above 0.90 power factor though THD is substantially higher than in the other mentioned converters.
Several switch-mode power supplies have been taught using diode bridge front ends followed by boost converters. For example. Smolenski, et al. in U.S. Pat. No. 5,019,952, Bruning in U.S. Pat. No. 5,003,454, Neufeld in U.S. Pat. No. 5,006,975, Williams in U.S. Pat. No. 4,940,929, and Wilkinson et al. in U.S. Pat. No. 4,677,366 each teach correcting the power factor by shunting a boost current following a diode bridge. Each of these teachings, however are limited to about 1,000 watts.
A somewhat different approach is taught by Wester in U.S. Pat. No. 4,193,111. Wester teaches connecting two transistor switching pairs in parallel with a diode bridge for shaping the input current so as to follow the input voltage by selectively switching the transistor switching pairs.
Still a different approach is taught by Severinsky in U.S. Pat. Nos. 4,943,902 and 4,964,029. Severinsky teaches the use of a diode bridge in the front end followed by a buck converter. This scheme, unlike those above requires that the voltage following the buck converter be less than its input voltage.
In U.S. Pat. Nos. 4,730,242 and 4,864,483, Divan teaches a DC-to-AC converter that relies upon resonating waveforms in a power converter for switching power transistors to reduce switching losses.
A circuit for improving the power factor to about 0.9 by increasing the conduction angle of the diodes making up a diode bridge by alternately switching two power transistors is set forth in IBM TDB vol. 32 No. 9A.
An AC-to-DC converter using a flyback converter is taught in IBM TDB Vol. 33. No. 8. Although the cited prior teachings improve or correct the power factor to a substantial degree, they all fall short of providing a switch-mode power supply capable of operating with a near unity power factor.
A proposed international standard by the International Electro-technical Commission (IEC) would require strict standards for harmonic currents generated by the front-end of electronic equipment connected to an AC utility. The electronic equipment's power factor is directly related to the level of harmonics in the input current. The standard, IEC 555-2, dictates absolute limits for harmonic currents that any electronic equipment can demand from (or inject into) the AC utility. According to the proposed standard, the input current of such electronic equipment must be nearly sinusoidal and in phase with the AC utility. The actual standards proposed by IEC 555-2 allow for decreased power factors for correspondingly decreased power requirements of the electronic equipment. If the proposed standard is passed, any electronic equipment shipped to the European Community, and world trade, will have to meet this standard.
Thus, what is needed is a switch-mode power supply that is capable of operating near the theoretical power factor limit of unity such that minimal harmonics are injected into or demanded from the AC utility as required by the proposed IEC 555-2 standard, and allowing an adjustment of the power factor and total harmonic distortion for various load power levels.